Carbon fiber is used in a variety of composite materials, by virtue of its excellent properties such as high strength, high elastic modulus, and high electrical conductivity. Since carbon fiber serving as a carbon material exhibits excellent mechanical properties in addition to electrical conductivity, carbon fiber has been utilized in a variety of fields. In recent years, in conjunction with developments in electronic techniques, carbon fiber has been considered a promising filler in conductive resin for producing electromagnetic wave shielding material or antistatic material, and has been viewed as a useful filler for electrostatic coating which can be incorporated into the resin. Also, by virtue of its chemical stability, thermal stability, and fine structure, carbon fiber has been considered a promising field emission material for use in, for example, flat displays. In addition, carbon fiber has been considered a promising wear-resistant, electrically conductive material for use in, for example, electric brushes and variable resistors.
Conventional carbon fiber; i.e., organic carbon fiber, is produced by subjecting organic fiber, such as PAN-, pitch-, or cellulose-based fiber, to heat treatment and carbonization. When such carbon fiber is used as a filler in fiber reinforced composite material, in order to increase the contact area between the carbon fiber and the matrix of the material, desirably, the diameter of the fiber is reduced or the length thereof is increased. As a result, the reinforcement effect on the composite material is enhanced. In order to improve adhesion between the carbon fiber and the matrix, the carbon fiber desirably has a rough surface rather than a smooth surface. Therefore, the carbon fiber is subjected to surface treatment; for example, the carbon fiber is oxidized by exposure to air at high temperature, or the surface of the fiber is subjected to coating.
However, conventionally, carbon fiber having a small diameter has been impossible to produce, since its raw material; i.e., organic fiber filaments, has a diameter of about 5 to about 10 μm. Furthermore, a limitation is imposed on the ratio of length to diameter (i.e., aspect ratio) of conventional carbon fiber. Therefore, keen demand has arisen for development of carbon fiber of small diameter and high aspect ratio.
When carbon fiber is incorporated into resin used for producing an automobile body, or into resin or rubber for producing an electronic device, the carbon fiber must exhibit electrical conductivity comparable to that of metal. Therefore, in order to meet requirements for a variety of electrical conductive paints and electrical conductive resins, carbon fiber serving as a filler material has been required to exhibit improved electrical conductivity.
In order to improve electrical conductivity, carbon fiber must be subjected to graphitization. Therefore, in general, carbon fiber is subjected to graphitization at high temperature. However, even when carbon fiber is subjected to such graphitization, the carbon fiber fails to attain electrical conductivity comparable to that of metal. Therefore, when the carbon fiber is employed in the aforementioned material, in order to compensate for low electrical conductivity of the carbon fiber, a large amount of the carbon fiber must be incorporated into the material. As a result, workability and mechanical properties of the material are impaired. In view of the foregoing, demand has arisen for further improvements to the electrical conductivity of carbon fiber, and enhancement of the strength of the carbon fiber by reducing its diameter.
In the late 1980's, studies were conducted on vapor grown carbon fiber (hereinafter abbreviated as “VGCF”) produced through a process which differs from that used for producing the aforementioned organic carbon fiber.
VGCF is known to be produced through thermal decomposition of a gas of, for example, hydrocarbon in a vapor phase in the presence of an organo-transition metallic catalyst. Through this process, carbon fiber having a diameter of hundreds of nm to 1 μm can be produced.
A variety of processes for producing VGCF are disclosed, including a process in which an organic compound such as benzene, serving as a raw material, and an organo-transition metallic compound such as ferrocene, serving as a metallic catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce VGCF on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which VGCF is produced in a dispersed state (Japanese Patent Application Laid-Open (kokai) No. 60-54998); and a process in which VGCF is grown on a reaction furnace wall (Japanese Patent No. 2778434).
The aforementioned processes can produce carbon fiber of relatively small diameter and high aspect ratio which exhibits excellent electrical conductivity and is suitable as a filler material. Therefore, carbon fiber having a diameter of about 100 to about 200 nm and an aspect ratio of about 10 to about 500 is mass-produced, and is used, for example, as an electrically conductive filler material in resin or as an additive in lead storage batteries.
A characteristic feature of a VGCF filament resides in its shape and crystal structure. A VGCF filament has a cylindrical structure including a very small hollow space in its center portion, and a plurality of carbon hexagonal network layers grown around the hollow space so as to form concentric rings.
Iijima, et al. have discovered a multi-layer carbon nano-tube, which is a type of carbon fiber having a diameter smaller than that of VGCF, in soot produced by evaporating a carbon electrode through arc discharge in helium gas. The multi-layer carbon nano-tube has a diameter of 1 to 30 nm, and is a fine carbon fiber filament having a structure similar to that of a VGCF filament; i.e., the tube has a cylindrical structure including in its center portion a hollow space extending along the filament, and a plurality of carbon hexagonal network layers grown around the hollow space so as to form concentric rings.
However, the above process for producing the nano-tube through arc discharge has not yet been put into practice, since the process is not suitable for mass production.
Meanwhile, production of carbon fiber of high aspect ratio and exhibiting high electrical conductivity through the vapor-growth process is thought to be feasible, and therefore attempts have been made to improve the vapor-growth process for the production of carbon fiber of smaller diameter. For example, U.S. Pat. No. 4,663,230 and Japanese Patent Publication (kokoku) No. 3-64606 disclose a graphitic cylindrical carbon fibril having a diameter of about 3.5 to 70 nm and an aspect ratio of 100 or more. The carbon fibril has a structure in which a plurality of layers of regularly arranged carbon atoms are continuously disposed concentrically about the cylindrical axis of the fibril, and the C-axis of each of the layers is substantially perpendicular to the cylindrical axis. The entirety of the fibril includes no thermal carbon overcoat deposited through thermal decomposition, and has a smooth surface.
Japanese Patent Application Laid-Open (kokai) No. 61-70014 discloses carbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, which fiber is produced through a vapor-growth process. According to this publication, a carbon layer obtained through thermal decomposition has a thickness of 20% or less the diameter of the carbon fiber.
When fine carbon fiber produced through the aforementioned vapor-growth process is employed as an electrically conductive material for producing electric products having sliding electrical contact points and involving friction or heat radiation, such as an electric brush and a variable resistor, demand has arisen for further improvements to tribological characteristics, electrical conductivity, and thermal conductivity of the carbon fiber.
An object of the present invention is to provide, on a mass-production scale, fine carbon fiber having a diameter of 500 nm or less and an aspect ratio of 10 to 15,000, and exhibiting excellent characteristics in terms of, for example, tribological characteristics, electrical conductivity, and thermal conductivity.